Development and application of fluorine doped bismuth vanadate reduced graphene oxide Nafion composite electrode as an electrochemical sensor for 4-chlorophenol

This study describes the synthesis of fluorine-doped bismuth vanadate (F0.1BiVO4) and its composite with graphene oxide (GO) to improve charge transport properties. Based on the structural and morphological analysis such as X-Ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), and RAMAN the composite of F0.1BiVO4/r-GO/Nafion was successfully prepared with no filth. It was used to selectively detect the environmental contaminant 4-chlorophenol (4-CP) on a modified glassy carbon electrode (GCE). The electron channeling ability of reduced graphene oxide (r-GO) with F0.1BiVO4 yielded a great electrochemical response (ER) in cyclic voltammetry compared to pure GCE and other modified electrodes. The differential pulse voltammetry response of 4-CP was highly sensitive with the detection of limit (LOD) of 0.56 nM and a wide linear response of 0.77–45.0 nM. Fluorine doping, in particular, was able to affect the crystal growth of BiVO4, which was the primary cause of the aforementioned improvement. On the other hand, r-GO acts as an electron bridge to improve charge transfer between electrolytes and F-BiVO4 due to its high electron transport rate. These results demonstrate the effectiveness of F0.1BiVO4/r-GO/Nafion/GCE for the electrochemical detection of 4-CP.

are regarded as the main disadvantages of enzyme-modified sensors.Moreover, electrodes modified with noble metal NPs (e.g., gold, silver, and platinum) are too expensive.One of the most important methods for increasing the sensitivity of sensors is the surface modification of a nano-sized core, which is a heterostructure with two or more materials at a nanometer scale in a hybrid structure.The unique physical and chemical properties of hybrid structures make them encouraging candidates for applications in various fields like biomedicine and electronics 12 .
In recent years, various metal oxides have received significant attention as electronic mediators with high potential for the fabrication of effective chemical sensors [13][14][15][16] .For instance, Uddin et al. 14 reported that ZnO/RuO 2 nanoparticles exhibited excellent sensitivity toward 2-nitrophenol.In another study, Ge-dopped ZnO composite on flat GCE was fabricated by electrochemical method.It has provided a better response compared to simple flat GCE or other electrodes in detection of 4-aminophenol 13 .Several reports have also shown that bismuth vanadate (BiVO 4 ) nanoparticle-based photoelectrochemical sensors exhibit a good response for the detection of nitrite anions (NO 2 − ) 17 and non-enzymatic H 2 O 2 photoelectrochemical sensors 18,19 .In addition to having a narrow energy gap of 2.4 eV, it has other unique characteristics, including excellent physicochemical properties, easy fabrication, good conductivity, low cost, chemical stability in aqueous solutions and non-toxicity 11,12 .Despite having a high electrical conductivity, pure BiVO 4 (P-BiVO 4 ) tends to agglomerate and reduces the fast charge and discharge capabilities due to its small specific surface area 20 .To overcome these drawbacks, many researchers have applied graphene oxide as a substrate for BiVO 4 .The large surface area of electrode materials has been regarded as important for enhancing the sensing capability.The development of reduced graphene oxide (r-GO), has been recently carried out to avoid the irreversible agglomeration.Gr has unique properties 21,22 , which provide r-GO with advantages such as high electron transport rate, high mechanical strength, and large specific surface area 23,24 .These advantages make r-GO to be a promising sensing substrate, on which hydrophilic groups serve as an electron bridge for enhancing the charge transport between BiVO 4 and electrolytes.
Up to now, several studies have confirmed that metal and non-metal doping can enhance electron transfer efficiency in BiVO 4 structure.For example, Rohloff and coworkers exhibited that the photo-electrochemical performance of Mo:BiVO 4 photoanodes achieved 21% 25 .Another study 3 has revealed that Au doping into BiVO 4 structure significantly improved water splitting performance.According to this study, Au plays a crucial role in visible light absorption and internal defects formation.Fluorine (F) doping in particular is capable of affecting the crystal growth of BiVO 4 , as well as providing an internal electric field that is helpful for separating photogenerated electron-hole pairs.Jiang and coworkers 26 reported that Fluorine-dopped BiVO 4 exhibited higher surface area, better light-absorbing performance, and as result of that lower bandgap energies, which enhanced photodegrdation of phenol to 97%.
In this work, a novel F 0.1 BiVO 4 /r-GO/Nafion composite was prepared using an effective three-step method (Fig. 1).First, F-BiVO 4 was synthesized via a solvothermal method using ethylene glycol solvent under a costeffective and mild reaction.This method enables the synthesis of materials with various crystal morphologies by adjusting synthesis parameters 27 .By incorporating F into BiVO 4 , defects are introduced into the lattice structure which leads to an increase in electron mobility and enhanced performance.Nonetheless, there is no report on the use of F as a template to synthesize BiVO 4 .Next, the r-GO, a 2D carbon nanosheet, was utilized as a substrate for immobilizing F-BiVO 4 .Then Nafion was applied to fabricated electrodes for stickiness (as chemical coating binder) of nanostructure F 0.1 BiVO 4 /r-GO onto GCE electrodes.One of the unique features of Nafion is its ability to facilitate proton transfer from its sulfonic groups to the perfluorinated hydrophobic backbone, which results in the formation of a highly conductive medium for protons to provide greater reticulation of the nanocomposites on the surface of the GCE.Finally, the modified electrode (F 0.1 BiVO 4 /r-GO/Nafion) was utilized as an ECS to detect 4-CP.The detection limit of F 0.1 BiVO 4 /r-GO/Nafion towards 4-CP was low, showing that it has the advantages of the combination of composite materials, such as fast electron transport rate, large specific surface area and high conductance.

Characterization
We recorded the FT-IR spectra on a thermoscientific NICOLET IR100 FT-IR Spectrometer which ranged from 400 to 4000 cm −1 using the KBr disk technique.A Renishaw instrument, model InVia reflex equipped with 532, and 785 nm lasers, was used to obtain the Raman spectra.The laser power was 10 mV.The spectral resolution was < 3 cm −1 .For all spectra, acquisition time was 60 ms (4 × 15 ms).The Raman spectra of each sample were filed at diverse areas of the sample.The phase formation of samples was investigated by performing PXRD through a X'Pert diffractometer of PAN analytical with monochromated Co-Kα (λ = 1.78901Å) radiation, in the 2θ range of 5° to 90°.The characterization of the electrode's surface structure was investigated through a scanning electron (FE-SEM, Te-Scan-Mira, Czechia, Europe) microscope together with an Energy-dispersive X-ray spectrometer (EDX, X-Max 50, Czechia, Europe).UV-Vis diffuse reflectance spectrometer (Agilent, USA, BaSO 4 is the reference) was employed for testing the UV-Vis absorption spectra of the composite.The electroanalytical experiments were performed through an EG&G PARSTAT 2273 electrochemical potentiated with a conventional three-electrode unit.The F 0.1 BiVO 4 /r-GO/Nafion-coated glassy carbon electrode (GCE) was the working electrode, and an Ag/AgCl electrode in aqueous 3.0 M KCl solution and Pt, respectively, were utilized as reference and counter electrode.The DPV scan was performed in a potential range of 0.0 to 1.8 V with a step potential of 8.5 mV, a pulse amplitude of 10 mV, a pulse width of 10 ms, a pulse period of 100 ms, and a scan rate of 0.01-0.07V/s.The scan rate was calculated as the potential increment per unit time during the DPV scan.

Preparation of P-BiVO 4 and F 0.1 BiVO 4
Spherical-like BiVO 4 was synthesized using a hydrothermal reaction, according to the prior reports 28 .For the typical preparation, we dissolved 2.0 mmol of Bi (NO 3 ) 3 •5H 2 O in 30.0 mL of ethylene glycol (EG) solution under continuous agitation at room temperature for 30 min.Next, we dissolved 0.234 g of NH 4 VO 3 in 20.0 mL of EG for 15 min under steady stirring.We added NH 4 VO 3 to the above mixture dropwise and a homogeneous solution was established following 1 h stirring.Then, we added NaF with the molar ratio of 0.5 wt% to the above solution and it was stirred for another hour.We kept the obtained solution in a Teflon-lined autoclave for 24 h at 180 °C.Following the reaction, we later collected the black precipitate through centrifugation using water and ethanol was used to wash it several times.We dried the obtained powders in a vacuum oven for 20 h at 60 °C and after that they were calcined at 450 °C for 5 h.

Fabrication of F 0.1 BiVO 4 /r-GO Composite
The synthesis of GO nanosheets was accomplished through the chemical oxidation of graphite flakes and then exfoliation, according to the improved Hummers' method 29 .The standard hydrothermal method was adopted to fabricate F 0.1 BiVO 4 /r-GO composites.In summary, we dispersed 0.05 g of F-BiVO 4 powder in 50.0 mL of deionized water and ethanol through ultra-sonication for 15 min.Afterward, we introduced 50.0 mL of GO suspension (2.5 mg/mL) into the F 0.1 BiVO 4 suspension dropwise, obtaining the weight ratio of GO to F 0.1 BiVO 4 at 0.0025:0.05.After stirring at 60 °C for 5 h, a homogeneous suspension was created.The F 0.1 BiVO 4 /r-GO suspension was then rinsed with ethanol and deionized water several times and dried in a 60 °C oven.The synthesis method's schematic view is presented in Fig. 1.

Fabrication of F 0.1 BiVO 4 /r-GO/Nafion
To prepare the working electrode, 2-10 mg of the prepared materials were dispersed in 1 mL of ethanol and isopropanol with a volume ratio of 1:1 and 15 µL of 0.1 wt% Nafion suspension under sonication for 30 min to form a homogeneous ink.Then, the bare GCE was polished with 0.3 μm Al 2 O 3 slurry and ultrasonically cleaned in distilled water and ethanol several times.Then, 2.5 µL of the prepared ink was loaded onto a polished GCE and dried.Different modified-electrodes were characterized by cyclic voltammetry (CV) in 5 mM [Fe (CN) 6 ] 3−/4− containing 0.1 M KCl.

Material characterizations
XRD in the 2θ range of 5° to 90° (see Fig. 2) was used to analyze the crystal phases of the BiVO 4 and F-BiVO 4 nanostructures.We can index the XRD patterns of P-BiVO 4 to the characteristic peaks (CPs) of monoclinic BiVO 4 (JCPDS 83-1697, m-BiVO 4 ), where the peaks located at 19.0°, 30.0°,30.5°,35.2°, 46.7°, 53.3°, and 59.5° belong to the (011), ( 112), (004), (020), ( 204), ( 116) and (132) planes, respectively.The pattern of F-BiVO 4 exhibited the main peaks of m-BiVO 4 .Meanwhile, the CPs of the BiVO 4 at 30.6°, 32.8°, and 39.9° shift to lower angles after modification with fluorine (see Fig. 2b).The observed expansion in the crystal lattice can be related to the well incorporation of F into the lattice of BiVO 4 , due to the fact F has a smaller ion radius (  [30][31][32] .Also, the XRD pattern of the F 0.1 BiVO 4 shows an increase in the peak intensity at 2θ = 27.5°,30° compared with that of BiVO 4 , suggesting that F-doping leads to the prompt of the crystallization of V 2 O 5 .Using the Debye-Scherrer equation, we obtained the average size of the crystal particles, which is equal to 45.5 nm.Moreover, the dipole moment (DM) of VO 4 3− tetrahedron deviated from zero to non-zero DM after BiVO 4 was doped with F, which can increase the separation of charge on photo-excitation and improve the electrochemical activity 20,26,32,33 .The 4-CP of GO (001) at around 11° almost disappears in the r-GO-BiVO 4 nanocomposite, which might be because of the low content of GO used in the nanocomposites 21,34,35 .
We recorded the FTIR measurements of P-BiVO 4 , F-doped BiVO 4 , and F-doped BiVO 4 /r-GO/Nafion composite in the range of 400-4000 cm −1 and Fig. 3 demonstrates the related results.Figure 2a demonstrates the characteristic bands of the BiVO 4 and F-doped BiVO 4 .The peaks around, 800 cm −1 , and 1100 cm −1 were attributed to the stretching mode of VO 4 -336,37 , and its branch at 650 cm −1 was ascribed to Bi-O 17,38 , and also peak at 745 cm −1 was because of V-F vibration 20,33 .The presence of V-F vibration corroborated the insertion of F atoms into the crystal lattice of BiVO 4 .Figure 3b presents the FTIR spectra of GO and F 0.1 BiVO 4 /r-GO/Nafion overlapped for an exact comparison.The GO spectra demonstrated numerous vibrational peaks that were strong, corresponding to different O functional groups.Because of OH stretching vibration, a strong GO absorption band was observed at 3410 cm −139, 40 .The CPs at 1725, 1371, 1211, and 1062 cm −1 could be attributed to carboxyl or carbonyl C=O stretching, carboxyl-OH stretching, C=C stretching, and alkoxy C-O stretching, respectively 23,41 .In the spectrum of the nanocomposite, F 0.1 BiVO 4 /r-GO/Nafion, there was a substantial reduction in the intensity of those peaks related to O functional groups, which revealed that GO was reduced into r-GO.The M-O-C bonds (M = V or Bi), usually appearing below 900 cm −1 , were observed around 810 cm −1 and demonstrated that metal oxide interacted with Gr 22,24 .
An efficient method for inspecting the crystallization, electronic properties of the materials and investigating the local structures is Raman spectroscopy.Figure 4a demonstrates the Raman bands and the presence of BiVO 4 in all of the samples that were synthesized.It was observed that BiVO 4 had a monoclinic phase according to the attributed stretching vibrations and twisting vibrations of the VO 4 3-tetrahedron 18,42 .The strongest Raman band around 831 cm was assigned to the symmetric V-O (A g ) stretching mode, but the weak Raman band at 706 cm was assigned to the antisymmetric V-O (B g ) mode stretch 19,43 .The Raman band near 372 represents the symmetric δ s (VO 4 3-) (A g ) bending mode and 331 cm −1 represents the antisymmetric δ as (VO 4 3-) (B g ) bending mode.The mode at 213.23 cm −1 was assigned as rotational and 126.58 cm −1 was assigned as translational.As shown in the inset of Fig. 3, the intensity of the bands increased in the Raman peak of F-doped BiVO 4 indicates (defect formation) that the VO 4 3− tetrahedron was weakly deformed, which was due to the fact that V ions were replaced with F ions in the F-doped sample, we could not observe extra peaks, but there was an enhancement in the intensity of the peak for F-doping, which was incorporated in the BiVO 4 and also increased the crystallinity behavior observed in XRD data.On the other hand, the Raman profile of GO was dominated by 2 CPs of carbonaceous materials located at 1351 and 1590 cm −1 , i.e., the D band peak because of the sp 3 defects and the G band peak, which could be attributed to in-plane vibrations of sp 2 C atoms 44,45 .The Raman spectrum of F-doped BiVO 4 /r-GO composite exhibited the same CPs as F doping in BiVO 4 and G and D bands of GO.Additionally, peak G in F 0.1 BiVO 4 /r-GO was relatively blue-shifted by 20 cm −1 in contrast with GO.Therefore, this clearly demonstrated that GO was effectively reduced into r-GO.The intensity ratios of the D and G bands (I D / www.nature.com/scientificreports/I G ) in GO and F 0.1 BiVO 4 /r-GO were determined 0.93 and 0.95, respectively.The I D /I G ratios for F 0.1 BiVO 4 /r-GO was higher than those for GO, indicating that a significant number of structural defects were introduced to the graphene lattice in the reaction.Conclusively, the larger I D /I G ratios in the F 0.1 BiVO 4 /r-GO compared to GO indicate an increase in the amount of smaller sp 2 domains and the revival of graphene network conjugation (rearomatization).Moreover, the revived graphene network size was smaller than that of the GO starting material.This effect gave rise to an increased I D /I G ratio in the F 0.1 BiVO 4 /r-GO composite materials.FE-SEM micrographs (see Fig. 4) show images of un-doped and F-doped BiVO 4 , and F 0.1 BiVO 4 /r-GO samples.Figure 4a shows that the structure of BiVO 4 has spherical-like and homogenous particles with diameters of 40-90 nm.The magnified images in Fig. 4b indicate that the spherical particles are formed by agglomeration of smaller polyhedral grain-like nanoparticles with size of 15 nm. Figure 4c and d show the SEM images of the F-BV sample which illustrates smaller nanoparticles than the P-BiVO 4 .After the composition of F 0.1 BiVO 4 with GO (Fig. 4e and f), the F 0.1 BiVO 4 nano-particles had a uniform distribution on the GO surface.The good distribution of nanoparticles on GO nanosheets could provide more active sites, which is beneficial for enhanced electrochemical performance.Additionally, the elemental distribution of C, O, Bi, and F in F 0.1 BiVO 4 /r-GO nanocomposite was achieved through energy dispersive spectroscopy (EDS) analysis.The results from mapping images in Fig. 5 and Table 1S confirmed that all of the essential elements were present and Fluorine atoms were distributed well in the BiVO 4 structure.

Fabrication of modified electrodes
To prepare the working electrode, 2-10 mg of the F 0.1 BiVO 4 /r-GO was dispersed in 1 mL of ethanol and isopropanol with a volume ratio of 1:1 and 15 µL of 0.1 wt% Nafion suspension under sonication for 30 min to form a homogeneous ink.Then, the bare GCE was polished with 0.3 μm Al 2 O 3 slurry and ultrasonically cleaned in distilled water and ethanol several times.Then, 2.5 µL of the prepared ink was loaded onto a polished GCE and dried.For comparison, a similar procedure was used to prepare F 0.1 BiVO4/GCE, GO/GCE, and BiVO 4 /GCE.Different modified-electrodes were characterized by cyclic voltammetry (CV) in "5 mM" [Fe (CN) 6 ] 3−/4− containing 0.1 M KCl.www.nature.com/scientificreports/ a relatively large oxidation peak with the potential of 0.78 V and the peak current of about 58 μA was observed (see Fig. 6b), revealing that F 0.1 BiVO 4 /r-GO/Nafion/GCE served as an efficient electron promoter for the 4-CP electrocatalytic oxidation.(Also, different percentages of fluorine dopant atoms were investigated.This is visible in Fig. 2S where the sample with a 10% dopant concentration showed the highest current among the other samples and was chosen as the optimal sample for further investigation).The DPVs of 4-CP with varying potential scan rates (0.01 to 0.07 V/s) on F 0.1 BiVO 4 /r-GO/Nafion/GCE were investigated to further understand the sensing mechanism of 4-CP (see Fig. 7a).In the Figure, a phenomenon we observed was that the oxidation peak potential did not shift much, and the peak currents gradually increased with increasing scanning speed.As can be seen in Fig. 7a,b, while there was an increase in scan rate from 0.01 to 0.07 V/s, the oxidation peaks current linearly increased.There was a linear dependence between the scan rate and the anodic peak current of 4-CP, and it obeys the following equation: The results show that the electrochemical response process of 4-CP onto the F 0.1 BiVO 4 /r-GO/Nafion/GCE sensor surface is controlled by adsorption.
The impact of pH on the CV responses of F 0.1 BiVO 4 /r-GO/Nafion/GCE was evaluated in 0.1 M Britton-Robinson buffer (BRB) in the pH range of 3.0 to 10.0 (see Figs. 7c,d, and 1S).Hence, pH = 7.0 is assigned as the standard pH for detecting 4-CP in this work.However, when the pH values are enhanced from 8.0 to 10.0, a decrease in the peak current occurs, which might be because of the electrostatic repulsion of anionic 4-CP which had negative charges on the surface of the sensor.Furthermore, the oxidation (Ep a ) peak potential became lower and lower by increasing pH.Also, we can express the dependence of pH on the shift in Ep a as follows 48,49 .This slope value is approximately the same as the theoretical Nernstian number of − 59.0 mV/pH, which demonstrated that the number of involved protons and electrons in the electron transfer process of 4-CP was equal.Therefore, it can be inferred that the electrochemical oxidation of 4-CP at the F 0.1 BiVO 4 /r-GO/Nafion/ GCE electrode is a one-electron and one-proton process and this is illustrated in Fig. 8  Under the optimal conditions, the analytical ability of the F 0.1 BiVO 4 /r-GO/Nafion/GCE sensor for detecting a series of 4-CP solutions with varying concentrations was investigated by the DPV method.As exhibited in Fig. 9a, a linear increase was observed in the oxidation peaks current of 4-CP (I p ) by increasing the concentration of 4-CP from 0.77 to 45.0 nM.As can be seen from Fig. 9b, we found that concentration and the oxidation peak current are closely related:  www.nature.com/scientificreports/ The detection limit for 4-CP was 0.56 nM (S/N = 3).Moreover, in Table 1, we compared the LOD values and the linear range with other previous reference for detecting 4-CP with electrodes based on GO.The data showed that the linear range was wider; the detection limit was lower, the sensitiveness was higher, the specific surface area was larger, and the electrocatalytic activity was higher for the 4-CP which was developed using the nanocomposite structure F 0.1 BiVO 4 and GO thin nanosheets, which were better than those of previously reported sensors.
The selectivity of the F 0.1 BiVO 4 /r-GO/Nafion/GCE sensor was evaluated by studying the influence of some potential interferents including inorganic ions and organic phenolic compounds, which were coexisted in the electrochemical detection of 4-CP in the subsequent DPV experiments.As shown in Fig. 8, 10-fold higher concentrations of ammonia, BPA, ethanol, hydroquinone, and phenol did not affect the detection of 4-CP.As shown in Fig. 10, the sensitivity of the proposed F 0.1 BiVO 4 /r-GO/Nafion/GCE towards 4-CP was approximately 10 times larger than interferents, indicating the proposed sensor had good anti-interference ability, and therefore, can be used to selectively determine 4-CP.
Additionally, The oxidation peak current of 1 nM 4-CP decreased with the successive potential scan on the F 0.1 BiVO 4 /r-GO/Nafion/GCE in BR buffer solution (Figs. 11 and 3S).This situation can be due to the adsorbed 4-CP oxidation product and the polymerization of 4-CP on the electrode surface (which blocks electrode surface and obstructs further oxidation of 4-CP) which is in accordance with previous studies.As can be seen, the oxidation peak current reduced by 13% after 12 cycles.According to result, the F 0.1 BiVO 4 /r-GO/Nafion/GCE modified electrode effectively prevent the surface fouling effect caused by the oxidation products of the 4-CP.According to the result above, the developed F 0.1 BiVO 4 /r-GO/Nafion/GCE modified GCE is a suitable platform for the 4-CP analysis with good stability and repeatability.To study the practicability of F 0.1 BiVO 4 /r-GO/Nafion/GCE modified electrode, real sample analysis was carried out using DPV technique.The standard addition method was followed to determine the 4-CP in tap water.The concentration of added PCMC and the recovery values are displayed in Table 2.The recoveries in the range of 101.10% to 102.73% were obtained, which suggested that the modified electrode might be applied for real sample tests.

Conclusions
In this study, the solvothermal method is applied to synthesize the catalyst F 0.1 BiVO 4 .In addition, an electrochemical method was applied to synthesize the F 0.1 BiVO 4 /r-GO/Nafion/GCE composite.Various techniques have been used to characterize the synthesized mixture for its morphological, structural and elemental properties.In addition, the 2D layered material synthesized with impregnated nanoparticles was exploited as an electrocatalyst for 4-CP detection.In addition, the quantitative determination of 4-CP through the proposed modified electrode provides a detection limit of 0.56 nM and a linear range of 0.77-45.0nM, a wide range.Thus, the modified GCE demonstrated high sensitivity, better stability, and a lower detection limit for 4-CP detection.These properties demonstrated the potential practical applications of the F 0.1 BiVO 4 /r-GO/Nafion/GCE for 4-CP sensors.Furthermore, the new sensor is efficient, environmentally sustainable, cost-effective and simple to manufacture.

Figure 8 .
Figure 8.The electron transfer mechanism and electrochemical reaction performance in 4-chlorophenol on the GCE modified with F 0.1 BiVO 4 /r-GO/Nafion.

Table 1 .
LOD comparison of F 0.1 BiVO 4 /r-GO/Nafion with other sensors reported on chlorophenol based martial.

Table 2 .
The recoveries of the prepared electrochemical sensor.